1. Field
The invention is in the field of silicon refining and is concerned with a process for producing a high-purity, polycrystaline silicon from the usual metallurgical grade or other impure silicon at relatively low cost for use primarily in photovoltaic (solar) cells.
2. State of the Art
Metallurgical grade silicon metal is produced in quantity commercially by the carbon reduction of silica (SiO.sub.2) in an electric arc furnace. The typical purity level is about 98% silicon. When this material is upgraded by refining to the point where total metallic impurities have been reduced to less than approximately 0.01 percent, it becomes a useful feed material for conventional single crystal growth processes and subsequent fabrication into photovoltaic cells.
Aluminum, iron, and calcium values are typically the principal impurities in arc-furnace, i.e. metallurgical grade, silicon. The amounts of these impurities in the silicon are lessened commercially by chemical reaction with gases to produce a low-aluminum grade of silicon widely used in industry.
Chemical reaction with gases produces either volatile compounds which are removed by evaporation, or insoluble compounds which float as a dross on the surface of the silicon metal in its molten state. A reactive gas, usually chlorine, but sometimes others, such as oxygen, carbon dioxide, water vapor, etc. is blown into molten silicon by means of a lance having its discharge end submerged in the melt. When used in the commercial production of metallurgical grade silicon, the gas is normally blown into the melt immediately following tapping of the electric arc furnace and in the refractory-lined ladle into which the tapped molten metal flows.
Some experimental work has been done in carrying chemical reaction further by the use of other gases, followed by some form of directional solidification, such as zone-refining, in attempts to achieve upgrading to the point at which the product can be used as feed material for single crystal growth processes. Yet, none of this has resulted in a commercial process.
Zone refining and other directional solidification techniques are widely used to produce semiconductor grade silicon, and their effectiveness is well known. In directional solidification, a liquid-solid interface is established and gradually moved through a mass of metal. The impurities selectively segregate in the liquid phase. When the mass of metal is completely solidified, the last portion to solidify will contain most of the impurities and is separated from the remainder of the mass as by sawing, leaving a mass of metal having greatly improved purity over the starting material.
3. Objective
A principal objective in the making of the present invention was to produce an intermediate grade of semiconductor silicon at relatively low cost and of sufficient purity to be useful in photovoltaic applications.